# Questions tagged [fft]

FFT is an acronym for Fast Fourier Transform. Use this tag for FFT-related questions. Try to also select a specific chip or software tag when possible.

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### Generating a sinewave on Matlab with specific harmonics [closed]

I need to generate a sinewave on Matlab so I can use the generated values, I need 1000 samples per cycle for grid simulation using micro-controller. I was able to generate a pure sinewave and it is ...
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### What is the unit of power spectral density?

I have an oscilloscope data, I did plot in time domain, so that unit is in volts (Y axis). After that I took FFT. So now what will be the unit in Y axis? If I will do 10 log 10 (y axis) can it is ...
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### Creating a Range Profile

I am relatively new to the radar/signal processing community. However, my team has been struggling in developing a range profile in our programming. Here are the details for a NI LabVIEW forum I ...
38 views

### How to get audio frequencies from mems microphone in PC oscilloscope

I'm fairly new to electronics/signal processing and I'm stuck on trying to obtain frequencies from an Adafruit SPW2430 MEMS microphone. I tried plugging the microphone to a Hantek 6022BE PC ...
130 views

### Generating and measuring a sine wave with THD below -80dB

For an experiment I need to generate a sine wave which has a THD below -80dB. I would like to do that using a band-pass filter connected to a function generator because the output of the function ...
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1 vote
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### Cross-correlation in time or frequency domain (FPGA)

I need to compute the complex cross-correlation between two signals a and b and the take only the maximum value of the cross-correlation, regardless of the lag time where this occurs. I can do it ...
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### LTSPICE: Dirty gate voltage and low gain - Hartley oscillator using MOSFET

I'm designing a Hartley oscillator using a MOSFET, but I have some issues. I've chosen L1 as 3 times L2 to obtain a gain of 3, however, it's more like 2.4. First of all, I can't understand why the ...
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### .four plot FF/t LTspice

I am trying to plot an FFT in LTspice using a .four command and the spice error log directly without using the FFT on LTspice (right click + view + FFT once the ...
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1 vote
82 views

### What determines amplitude resolution of oscilloscope FFT?

I am studying my oscilloscope's FFT measurement capability (Rigol DS1054Z. Links to datasheet and user guide), and do not understand what determines the amplitude resolution capability of the FFT ...
127 views

### What is happening when a signal is acquired at exactly Nyquist frequency?

So I am doing some some simulations with 1V sinusoidal signals, and a 1 second acquisition at sampling frequency of 100 kHz. Now I am slowly increasing the frequency of the sinusoidal signal. Trying 1 ...
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### Mismatch in port width for Xilinx FFT IP config field

I am instantiating the FFT IP core from Xilinx with the following parameters (all others are left to default): Number of channels : 4 transform length : 1024 target clock frequency : 250 This ...
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### Windowing FFT: spectral leakage vs ENOB

I am using Cadence to perform full speed testing of a 4-bit SAR ADC using the FFT and non-coherent sampling. I've obtained the ENOB and the spectra of the output signal using first the rectangular ...
• 1,117
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### Why am I getting linear phase?

I'm trying to plot the phase of Fourier transform of the function below: \begin{gather*} x[n] = 2e^{-0.9|n|} \: \: n \in [-5,5]\\ x[n] = 0 \:\: n \notin [-5,5] \end{gather*} which is equal to the ...
1 vote
80 views

### Numpy giving the wrong Fourier transform of sine functions

I'm using numpy's FFT to calculate the Fourier transform of $\cos(\omega_0t)$ and plot it. By using the definition we can derive its Fourier transform as below: X_3(j\omega) = \pi (...
160 views

### How can we obtain the input signal given the output signal and transfer function?

From an electrical circuit, I have the transfer function as: $$H(s) = \frac{R}{sRC + 1}$$ $V$ is the output and I is the input so we can write: $$V(s) = I(s) H(s)$$ or $$V(jω) = I(jω) H(jω)$$ ...
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### Reason for frequency steps shown in Simulink FFT analysis tool

While using the FFT in Simulink tool, I have observed that if more number of cycles are selected then frequency steps in which harmonics percentage is shown is also divided accordingly. For example: ...
41 views

### How to use DFT to find harmonic content of an AC waveform?

I am kind of new to this coding and simulation environment. For my first project I need to take samples of current signals from power line and find out the harmonic contents using DFT. I haven't done ...
1 vote
96 views

### Voltage RMS and Voltage Spectral Density from ADC Sample are Not Consistent

I have time series data from a 100 MS/s ADC, and I would like to calculate the RMS noise and the voltage spectral density (VSD), and see consistency between the two. I'm using a 16-bit ADC, and the ...
• 11
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### Uniform circular array (UCA) beamforming using FFT

As far as I know, conventional beamforming using FFT can return a range-angle map. However, this only works with a uniform linear array (ULA). I'm wondering if the same approach works for UCA. I guess ...
38 views

### What should be my sampling frequency for a peak detector output where it is extremely important for me to capture the first time the peak occured?

Following is the output of the peak detector: The amplitude is not important to me, however the time when the peak occurs is extremely important. So my microcontroller will have an algorithm like ...
78 views

### 5G NR and OFDM modulation with IFFT

I'm studying 5G NR and to understand better how it works I'm trying to implement it in Simulink using the built-in OFDM block. From what I've understood, the numerology in 5G basically set the ...
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### OFDM IDFT implementation

i have some doubts with the implementation of the IDFT in OFDM systems. The question concerns the expression of the IDFT of the OFDM signal. During the symbols period $T_s$ we have the following ...
183 views

### How to calculate subcarriers bandwidth and cyclic prefix length in an OFDM using the IFFT

I'm trying to understand how OFDM works thanks also to the Simulink OFDM block. Now, I have understood that the OFDM modulation "convert" a faster bit rate using a certain bandwidth into a ...
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216 views

### How to convert amplitude (V) to dB (V^2/Hz or V/sqrt(Hz) or V?)

I took the FFT of a signal and am currently plotting amplitude (V) vs. frequency (Hz). I'd like to change the Y-Axis units to dB. How would I go about doing this? What would be the units? It is from ...
159 views

### How to detect dog bark? [closed]

I need to design a small dog bark detector. It should be as cheap as possible (10\$ manufactured unit.) It should be accurate because it can be used anywhere in noisy places. The aproach I am ...
62 views

### What happens if the signal frequency exceeds the sampling rate?

As the title states, I was wondering what happens to the frequency contents of the sampled signal of a pure sinusoidal signal that exceeds the sampling rate. I am familiar with the shannon theorem and ...
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71 views

### FFT analysis of summed signal in LTSpice

I know that simulating FFT analysis of current/voltage across a single element via LTSpice is quite straightforward. I have simulated the summation of three distinct currents in the time domain. I'd ...
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### Meaning of dB units in periodogram (FFT)

When I calculate spectrum of a voltage signal using FFT and then I calculate periodogram using $$S_k=10\cdot log_{10}(\frac{1}{N}|X_k|^2)$$ where $X_k$ if the result of FFT and $N$ is number of ...
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### What causes the small peak at the start of the spectrum in a spectrum analyzer?

There is a "DC" peak that is at the start of the spectrum. I'm studying the differences between superheterodyne and FFT spectrum analyzers and I'm wondering if the peak is caused by the same ...
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1 vote
226 views

### Scale dB SPL based on mic sensitivity

I have a measurement performed using GRAS 46AE microphone calibrated at 410mVrms at 114dB (250Hz). I have a .wav file recorded and performed stft and obtained the magnitude. Now I would like to scale ...
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### What is derivative of FFT outputs with respect to time?

I am quite new Digital Signal Processing.I am trying to implement anti-cogging algortihm for my permanent magnet synchronous motor (=PMSM) control algortihm. I follow this documentation I collected ...
79 views

### Is AC coming from a DC powersupply normal?

I recently bought a Oubel Laboratory Power Supply 60V/5A, that I'd like to use for hardware diagnosis. When measuring the - and + outlets, the oscilloscope picks up ac voltage, that propagates through ...
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### FMCW radar FFT(Fast Fourier Transform)

I am trying to design a simple FMCW radar for the ideal case. Below are steps I planned. Transmit signal, Tx = cos(T) receive signal, Rx = cos(R) Mix transmitted signal and received signal, Sx1 = cos(...
337 views

### Is it possible to do an FFT on the output of a 4 bit ADC?

I made a simple 4 bit flash ADC on a breadboard, and I want to run an fft on the output. Is it possible to do an FFT on data that is only 4 bits in resolution.
54 views

### The effects of frequency resolution in FFT on the frequency spectrum

This is a section for the parameter study of my thesis (basically: why and how I did what I did in the experiments). Currently, I have to choose the fit bandwidth and FFT lines combination for the ...
510 views

### Simple FFT with DSP on STM32F407VG: How to interpret results and clear noise

It's my first project with STM32 (board stm32f4-disc1). I'm trying to create a simple spectrum analyzer for analyzing a signal (400-7000Hz, 0-3000 mV). My goal is to catch an event when the signal is ...
145 views

### How can I see the magnitude and frequency of a signal (DFT) in PLECS?

I am trying to obtain the Fourier transform of a signal by using the DFT of PLECS but I got some weird results. Here is a very simple sinewave generator and the DFT block: DFT_plecs_1 Here are the ...
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### Frequency of a load curve signal

I have measured a charge and discharge curve of a capacitor with resistor and I want to calculate the frequency of this signal (for a low pass filter). Am I on the right way if I calculate 1 divided ...
160 views

### Overtones(?) seem to be showing up in FFT of audio signal

I am processing audio data using a Teensy 3.2. I have a microphone attached to it, and am playing music in my room using some powerful speakers. When I look at data from the Fourier Transform of the ...
1 vote
401 views

### Bucketing FFT into one-third octave bands

I'm trying to understand how to take the results of an FFT calculation and bin them to one-third octave bands. As my questions aren't about the code but about fundamentally understanding the arguments ...
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519 views

### Relation between Fast Fourier Transform and Laplace/Continuous Fourier representation

I am trying to identify a system by means of its differential equation (i.e., Lapace representation). I put together a rather straightforward regression algorithm (similar to Proni's method for ARMA) ...
• 8,146
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### FFT of time-compressed signal not outputting correct amplitude spectrum

Given a signal g(t) <--> G(f), a time-compression g(at) <--> 1/(abs(a)) * G(f/abs(a)), but not with the FFT as I've found out. Performing the FFT on g(a*t) doesn't scale the amplitude of ...
1 vote
149 views

### Low-pass filter on PDM data provides data that does not have a FFT

I'm trying to convert PDM data into analog form by passing the data through a low-pass filter in Matlab. My approach is fairly simple and matches the answer given in this Electrical Engineering Stack ...
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94 views

### FFT Frequency Bin - Impact on Energy Totals

If I reduce the number of samples used for an FFT (increasing the width of the frequency bins), is it correct to say that the energy in two bins with smaller frequency bins (eg. 10kHz-20kHz and 20kHz-...
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### Calculate receiver dynamic range (understanding the effect of FFT processing gain)

I'm attempting calculate the dynamic range of a receiver. That is, the ratio between the maximum and minimum detectable powers at the reception antenna. The receiver is composed of (in order) an ...
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### FMCW - FFT - Range resolution

with a prototype of FMCW radar I'm trying to resolve the distance to the target The Audio card characteristics are as follows: Fs= 44100Hz So the FMCW radar range resolution is RR=440MHz/c = 0.34m Ts=...
54 views

### Negative Freqs from FFT in Laplace Circuit Analysis

Do I use the absolute-value of the FFT frequencies when using the Laplace equations for calculating the impedance of an RLC circuit? I'm currently working on something where I need to numerically ...
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### FM demodulator LTspice

I am now designing an FM demodulator circuit. And the first step is turning FM into AM, so I could use envelope detector afterwards. Modulator circuit is designed using Colpitts topology. So frequency ...
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94 views

### Would unstable sample rate affect spectrum after FFT?

The system samples analog input and then conduct FFT on the samples and then display the spectrum. Currently, the issue is the displayed spectrum line position is keeping shift between each scan. ...
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